To increase the overall efficiency of photovoltaic devices and extract the maximum amount of energy from solar radiation, researchers have investigated various multi-color photovoltaic devices. These multicolor photovoltaic devices can be divided into two general categories. The first category covers monolithic multicolor solar cells. A monolithic multi-color solar cell is a solar cell which has distinct regions optimized to absorb different portions of the solar radiation spectrum in a single device. U.S. Pat. Nos. 4,404,421 and 4,451,691, incorporated herein by reference for all purposes, describe suitable monolithic devices. Although these monolithic devices are attractive from a system and manufacturing point of view, they will require considerable materials research to bring them to commercialization.
A second approach involves tandem mechanically stacked two-color solar cells. These devices comprise independent photovoltaic devices which optimized to different portions of the solar spectrum and are mechanically and electrically interconnected. These tandem mechanically stacked two-color solar cells offer a shorter path to commercialization primarily because the low-band gap cells, such as silicon solar cells, are already developed cells. U.S. application Ser. No. 645,456 filed Aug. 28, 1984, incorporated herein by reference for all purposes, describes a suitable high-band gap solar cell. Examples of high-band gap solar cells are GaAsP or AlGaAs or GaAs solar cells, and the like.
These mechanically stacked cells often fall into the category of solar cells known as concentrator solar cells. A concentrator solar cell is a high efficiency solar cell which utilizes some sort of a focusing optics to concentrate solar radiation from a strength of one sun to many suns, i.e., on the order of 50 to 1000 or more suns. The concentration of the solar radiation permits the solar cells to produce a greater amount of electricity per unit area than lower efficiency flat plate solar cells. This makes them especially useful for space applications where weight is of great concern and in jobs which require maximum electrical output with a minimum amount of surface area. However, a drawback to concentrator solar cells is a means for interconnecting the two mechanically stacked solar cells and dissipating the heat generated by the concentration of the solar radiation. Thus, it would be highly desirable to have a mechanically stacked apparatus which can interconnect two solar cells while minimizing the effects of heat generated by the concentrated solar radiation.
In conventional mechanical stack designs, in particular, those using thin top cells, the heat generated in the top cell must be transmitted through the transparent adhesive bonding the two cells together. This can lead to undesirably high cell temperatures. To avoid this difficulty, it would be highly desirable to have a package design wherein heat spreaders incorporated therein are used both for the bottom and top cells. A further advantage would be to incorporate a wafer for the top cell that is thick enough to conduct the heat laterally to the second heat spreader. A still further advantage or object would be to have a design which isolates the cells so that the effects of thermal expansion are reduced or minimized.
In mechanical or monolithic cell designs, the top and bottom cells must generally be current matched or the performance of the device is limited by the cell having the lower current. Since current matching different bandgap solar cells can be extremely difficult, it would be desirable to have a package which permits voltage matching of the two cells. Voltage matching is beneficial because the voltages of the cells change very little with variations in solar spectrum or with the cell degradation with space radiation damage. Thus, it would also be highly desirable to have a package design which can dissipate the heat and permit the easy wiring of numerous mechanically stacked cells into a module wiring configuration for voltage matching instead of current matching.